For the experimental examination of the radial transport of plasma impurities, i.a. a small amount of a plasma impurity is introduced into the plasma at the plasma edge, and then, the development in time of the spectral lines of different degrees of ionization is measured. In such experiments, which are described e.g. in “New diagnostics for physics studies on TEXTOR-94”, Review of Scientific Instruments, Vol. 72, Pages 1046-1053, the particles that constitute the impurities in the plasma—hereinafter referred to as impurity particles—advance into regions of increasingly higher electron temperature, starting at the edge of the plasma towards the centre of the plasma, and are gradually ionized to ever higher degrees of ionization. The impurity particles are then present in the form of ions. At the same time, these ionized impurity particles—hereinafter referred to as impurity ions—are excited to emit characteristic spectral lines. In the experiment, spectral lines of higher degrees of ionization appear delayed in time compared to those of lower degrees of ionization. This delay between the appearances of two spectral lines of different degrees of ionization, Z1 and Z2, is a direct measure for the transport time that the impurity particles require for transport between those radial positions, r1 and r2, at which the emissivities of the observed spectral lines are located. The accuracy of the described method is dependent on the ability to observe simultaneously as many different spectral lines of different degrees of ionization as possible, with a good signal-to-noise ratio and sufficient time resolution.
Preliminary tests showed for typical conditions in a fusion experiment that, as a rule, a time resolution of 1 ms is necessary in order to be able to resolve sufficiently the delay between spectral lines of different degrees of ionization by means of measurement technology. Continuous measurement should be possible over the total discharge duration of a typical plasma (6 to 10 seconds) in order to be able to record all processes during the discharge that are interesting to physics.
The discrimination between spectral lines and the background as well as the separation of different spectral lines can only be achieved, as a rule, by using suitable spectrometers having sufficient wavelength resolution. The method's applicability to as many different relevant plasma impurities as possible, after all, determines the wavelength range in VUV (Vacuum Ultraviolet, approx. 10 nm-100 nm) which is to be observed, since in this range, the impurities relevant to fusion experiments show many of their strongest spectral lines.
The requirements with regard to measurement technology and especially regarding the spectrometer used for the planned tests can therefore be summarized as follows:                a) The spectrometer should be able to work over a broad wavelength range in the VUV (10-100 nm).        b) The spectrometer should be able to perform measurements with a high degree of time resolution, namely at least 1000 complete spectra per second.        c) The efficiency of the spectrometer should be high, since the signal-to-noise ratio also depends on the photon statistics.        d) The spectrometer should have sufficient wavelength resolution (line separation).        e) The spectrometer should have a wide dynamic range since the intensities of the individual spectral lines vary greatly.        
The spectrometer concept “SPRED” (Survey Poor Resolution Extended Domain) which is presented in works by Fonck et al., Appl. Optics Vol. 21, page 2115 et sqq. (1982), as well as by Stratton et al., Reviews of Scientific instruments Vol. 57, page 2043 et sqq. (1986), was found to be the closest state of the art at the moment able to meet the above-mentioned requirements.
The spectrometer concept may be summarized briefly as follows: the centerpiece of the spectrometer is a diffraction grating by Jobin-Yvon with the following properties:    I. Oblique incidence of light at approx. 70 degrees from the normal of the grating for obtaining sufficient reflectivity of the coating of the grating in the range of under 50 nm.    II. Toroidal grating substrate for reducing the geometric loss of light through astigmatism. Efficiency is thus enhanced.    III. The grooves of the grating are produced by ion etching or holographically. Thus, a high degree of efficiency is obtained in the first diffraction order while at the same time, higher diffraction orders are suppressed. Furthermore, a reduction of image defects is achieved and a sharp spectrum in a plane of 40 mm width is obtained.    IV. The grating surface is coated with gold in order to improve efficiency in a wavelength range of under 30 nm.    V. The lengths of the two arms of the spectrometer (which is to be understood as being the distance entrance slit—grating or grating—detector) are chosen to be approx. 30 cm each, so that, for the size of gratings and groove densities that can be produced today, the result is an instrument with a large wavelength range of sufficient wavelength resolution and large aperture (f/30), and thus with a high degree of efficiency.
The diffraction grating creates images of the entrance slit in the exit plane of the spectrometer of a scale of approx. 1:1.
An open MCP detector (“Multi Channel Plate”) is used in the exit plane of the spectrometer for converting the VUV photons into visible light and amplifying the signals at the same time. The whole spectrometer is operated in a vacuum because radiation in the wavelength range of 10-100 nm is absorbed by all gases and all materials. The operation of the open MCP detectors furthermore necessitates a pressure of under 10−6 mbar, so that a UHV set-up is necessary for the whole spectrometer (UHV: Ultra High Vacuum).
The technical requirements desired according to a) to e) make it necessary to make specific improvements in certain points on the devices known from the above-mentioned state of the art.
The object of the invention is the creation of an apparatus of the kind mentioned at the beginning with which the transport of impurities in a plasma can be measured more accurately than was possible up to now.
The object is achieved by an apparatus having the characterizing features of the main claim. Advantageous embodiments result from the dependent claims.